This invention relates generally to television receivers and particularly to the horizontal scansion systems thereof.
In the typical television receiver, a transmitted signal bearing information components of picture, sound and deflection synchronization information is received by an antenna and processed by a tuner and intermediate frequency amplifier to a level sufficient to permit recovery of the modulated components. The latter take the form of a combined train of pulses at vertical and horizontal scansion frequencies. A cathode ray tube (CRT) display device is caused to be simultaneously scanned in the vertical and horizontal directions by individual vertical and horizontal scansion systems within the receiver.
In both the horizontal and vertical systems, a local oscillator generates a scansion signal which is increased by appropriate power amplifying circuitry to a level sufficient to drive electromagnetic yoke windings situated on the CRT. Proper display of the picture components within the signal requires that both the vertical and horizontal scansions be appropriately timed to the incoming signal information. In the vertical deflection or scansion system (operative at approximately 60 hertz), the vertical scansion synchronization pulses are usually applied directly to the vertical oscillator triggering it and causing it to operate at the desired frequency and phase.
While this method of synchronization has proven satisfactory for the vertical scan system, the higher frequency (approximately 15 kHz), horizontal scansion system is generally synchronized in a different manner. In most horizontal scan systems a local automatic phase control loop is operative upon the oscillator. Such systems generally include a phase detector or multiplier which responds to the reference synchronization pulses and a feedback sample of horizontal oscillator output. The phase detector compares the reference and oscillator signals and generates an error signal representing the deviation of oscillator frequency and phase from that of the reference pulses. The error signal is coupled to a low pass filter which minimizes the effects of signal noise and determines the response speed of the APC loop. The filtered error signal is coupled back to a voltage control point within the horizontal oscillator completing the loop and affecting frequency control.
Automatic phase control systems operative upon the horizontal oscillator, are well known in the industry. However, while their use has proven generally satisfactory under most operating conditions several often serious shortcomings do arise. For example, the system requirements of adequate oscillator pull-in range and optimization of system speed may be conflicting.
Oscillator pull-in range is the maximum frequency difference between the oscillator and reference sync pulses which the system can overcome or correct. Generally, pull-in range is determined largely by the gain of the APC loop together with the transfer characteristic of the APC filter. In many systems the limit of synchronization is that phase difference which produces a quadrature relationship between oscillator and sync reference.
In most instances system speed is determined largely by the bandwidth of an APC filter within the loop. The filter is selected to minimize noise effects on oscillator phase. However, narrower bandwidths reduce system pull-in range. Because the two system requirements are conflicting, typical receiver APC systems represent a compromise in which oscillator pull-in range is selected with an eye toward oscillator drift characteristics resulting in a wider system bandwidth than otherwise desired.
Another approach to the problem is to sense the absence of synchronization and activate an auxiliary oscillator which modulates the error voltage applied to the horizontal scansion oscillator causing it to "sweep" through a range of frequencies. If during its frequency variation synchronism occurs, the auxiliary oscillator is deactivated and synchronization is maintaind by the "normal" APC action.
Such systems simultaneously provide the advantages of extensive pull-in range and minimized static phase error because the APC filter characteristic and loop gain are selected for minimum static phase error. The pull-in range is determined by the frequency excursion of the scansion oscillator which is substantially independent of the APC loop filter and gain characteristics. While providing a high performance alternative to the compromized APC loop such systems are generally expensive and complex since an additional otherwise unnecessary oscillator is used to "sweep" the horizontal scansion oscillator. Further the circuitry required to sense the presence or absence of scansion synchronization is generally complex, expensive and often unreliable.